Sunday, June 28, 2009

Carburettor Madness

  • Rebuilt the carburettor (Weber DGV)
  • Fixed fast idle
  • Got normal gas mileage (~23 mpg instead of 7)
  • Misfiring and backfiring resolved
Diligent readers will have noticed that it's been a while since I've last posted. There are various reasons for this -work, travel, family, etc, I'm writing this in Poland- but the most pressing one would have to be that when I replaced the B's exhaust, as described in the previous post, I killed the car.
It just so happens that the various systems of this B were working together in perfect harmony. I was always aware that the engine was running rich (7mpg is not very good, not even for an old MG), and that the exhaust had a massive gap under the hood and was sending exhaust fumes into the cabin. But I didn't know that the exhaust gap was indicative of a clog further down the line, and the exhaust fumes not being vented were choking the engine, which was compensated for by having a richer fuel mixture. When I replaced the exhaust system the engine was free to breath again, and the excess fuel was burning off as it should. However, this resulted in an engine refusing to go lower than 3000 rpm (and would stay high if you tapped the accelerator), an exhaust manifold turning bright orange, and flames shooting out the carburettor and the muffler (worse than before). My car was shitting itself while throwing up, but still devouring gas. It was bad.

There were several problems with the carburettor after I rebuilt it, mostly linked to the previous owner not having the right settings and overcompensating for the asphyxiated engine. Rebuilding a Weber is quite straight forward (I bought a manual but have yet to read it); essentially just grab a screw driver and pull stuff out. In a next post I will post pictures with descriptions (once again, I'm in Poland right now). Here's what I've learned, and some tips on what to do when rebuilding your carburettor (specifically, the Weber DGV):
  • Unless you really need to change everything, don't bother with a rebuild kit. Buy the gaskets separately, and get all the rest from a local hardware store. Rebuild kits are about $35 for stuff you can get for less than $15. Also, in my case the parts were of inferior quality (previously metal parts are now plastic or even paper!). This brings me to ...
  • Buy a new power valve. I guarantee you, if it wasn't already broken you've broken it now. A broken power valve is what caused my engine to race and not idle below 3000 rpm (the power valve activates circuits supplying fuel for conditions requiring more fuel than at idle). It's an easy replacement I highly recommend, but is not included in most rebuild kits. It costs about $20.
  • If your car comes with a vacuum advance vacuum port on the distributor, keep in mind that ported vacuum from a Weber acts differently than manifold vacuum, the system the distributor was designed for. The engine goes crazy when I connect the vacuum port from the Weber to my stock distributor (the original one with an amplifier in my '77). The engine runs fine without it, so I'm leaving it disconnected for now. UPDATE: This does not happen anymore. In fact, connecting the vacuum port to the distributor actually reduced misfiring. I think there were too many other problems at the time I wrote this article. Alternatively, you could buy a different intake manifold (a non-Weber manifold) which has more vacuum manifold ports (you'll see what I mean if you care about vacuum). As I'm replacing most things, I'm replacing the distributor entirely at some point.
  • Do all fine-tuning before putting the carb back on. On my B the fine-tuning screws face the wrong way, towards the engine, making fine-tuning a pain in the butt. Also, with the carb out you can see what the fast idle screw is doing to the throttle, and how it's placed with respect to the fuel lines of the various circuits. I'll go into more details on this later.
  • The float was another source of frustration. Too high, and fuel will spill over the holding tank and go everywhere, which could result in a mini aurora-borealis under the hood. Too low, and the car will never run on it's own. Weber manuals describe correct heights of floats, and I strongly recommend you stick to them. Unfortunately my float did not appear in any manual, and as it was a brass one (not plastic like the new ones) I wanted to keep it. So I just fiddled with it a few times till fuel wasn't overflowing or starving the engine. This took a while, and was very frustrating.
  • British fuel pumps are garbage. One minute they're too weak, the next they're so powerful the intake needle won't close and fuel shoots out everywhere. I recommend buying a fuel pressure regulator and setting it for 3.5 psi.
  • WEAR SAFETY GOGGLES!!!! Flames shooting out the intake almost cost me all my facial hair. I guess I'd recommend at the very least keeping the air filter and cover on while doing any work with an engine running... Also, I had to often remove the fuel line while it still had some pressure, resulting in fuel shooting out. Fuel in your eye can result in blindness! Wear goggles! (Well, atleast while doing stuff with fuel and oils.)
The trouble with carburettors is that they are very fine-tuned pieces of equipment. Screws slightly off can result in complete chaos. For example, if your mixture is too lean the engine won't start or stay on at all. Too rich, and you could blow off more than just your eye-brows. In either case, long term usage of bad tuning can result in severe engine damage (slightly off is OK).

General Weber tuning instructions can be found here. These are very good instructions that helped me get things started again. Remember to adjust the timing after this.

Some documents of interest:
Quick trouble shooting guide (did not help me)
Complete Weber installation guide

Note (from below): These files and links are posted for informational purposes only. No profit what so ever is generated from these files. These files can be found everywhere on the internet and are simply collected here for reference purposes.

UPDATE: I have done many more things to the carburettor. Click on the 'weber' label on the right for all updates.

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